Created using remix-ide: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://remix.ethereum.org/#version=soljson-v0.8.26+commit.8a97fa7a.js&optimize=false&runs=200&gist=

HashtableExample.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.8.2 <0.9.0;

struct Hashtable {
    mapping(address => uint) valueMap;
    address[] keys;
}
function put(Hashtable storage self, address key, uint value) returns(bool) {
    if(self.valueMap[key] != 0) {
        return false;
    }
    self.valueMap[key] = value;
    self.keys.push(key);
    return true;
}

using {put} for Hashtable;

contract HashtableExample {

    Hashtable tokenValues;

    function mint(address receiver, uint amount) public returns(bool) {
        return tokenValues.put(receiver, amount);
    }

    function totalSupply() public view returns(uint) {
        uint total = 0;
        for(uint i=0; i<tokenValues.keys.length; i++) {
            address user = tokenValues.keys[i];
            total += tokenValues.valueMap[user];
        }
        return total;
    }
}